The FDA's finding that the Impax/Teva generic Wellbutrin XL 300 is not the bioequivalent of the GSK product has narrowed the generic antidepressant market to four distributors. The FDA's findings followed an agency-sponsored study of 24 adults comparing the two products. The study found that absorption rates didn't match, meaning the drug didn't enter the bloodstream at the same rates or to the same degree. Impax makes the generic and Teva distributes it, both functions stopped at the FDA's request September 28. The findings do not affect the 15 mg version. The FDA has also asked generics makers Anchen, Actavis, Watson and Mylan to test the bioequivalence of their XL 300 generics and deliver results by March 2013.
A stream of complaints that began rolling into the FDA in 2007, soon after the agency approved the 300mg dose in December 2006, kicked off the investigation. An agency FAQ document about the findings said it received 85 post-marketing reports that indicated that almost half of the patients who switched from the branded version to the generic said the drug wasn't working or had adverse events that were in line with what was noted on the label. FDA said the reports said that these same patients were fine when they jumped back to the branded version.
Testing occurred in two phases -- the agency first tasked Impax and Teva with doing a head-to-head comparison in 2007 using patients who reported problems when they changed their drugs, but the study was cut short because the companies said they could not get enough patients. The FDA then launched its own study, with a slightly different parameter, and recruited 24 adults who had not been treated with the drug.
The agency approved all of the XL 300 generics based on the bioequivalence of their respective 150mg versions, which the agency noted in its write-ups is not unusual. FDA explained that it opted not to require the five companies to test the higher dose because of “concerns that this higher strength of the drug could cause seizures in healthy adult volunteers,” and opted to use what it calls a waiving-up approval, which means using results from the lower dose to approve a higher dose where there's concern that it could cause serious adverse events in healthy adults.
Teva said by email that it would not comment as to whether it was considering introducing its own version of the antidepressant.